In 3GPP (3rd Generation Partnership Project) which is a project aiming to standardize a mobile communication system, it has been studied to introduce device to device (D2D) communication as a new feature after Release 12 (see Non Patent Document 1).
In D2D communication, a plurality of neighboring user terminals performs direct communication without passing through a base station. That is, a data path of D2D communication does not pass through a base station. On the other hand, a data path for normal communication (cellular communication) in a mobile communication system passes through a base station.